Tectonic setting of metamorphism and exhumation of eclogite-facies rocks in the South Beishan orogen, northwestern China

Abstract

High-pressure metamorphic rocks occur as distinct belts along subduction zones and collisional orogens or as isolated blocks within orogens or mélanges and represent continental materials that were subducted to deep depths and subsequently exhumed to the shallow crust. Understanding the burial and exhumation processes and the sizes and shapes of the high-pressure blocks is important for providing insight into global geodynamics and plate tectonic processes. The South Beishan orogen of northwestern China is notable for the exposure of early Paleozoic high-pressure (HP), eclogite-facies metamorphic rocks, yet the tectonism associated with the HP metamorphism and mechanism of exhumation are poorly understood despite being key to understanding the tectonic evolution of the larger Central Asian Orogenic System. To address this issue, we examined the geometries, kinematics, and overprinting relationships of structures and determined the temperatures and timings of deformation and metamorphism of the HP rocks of the South Beishan orogen. Geochronological results show that the South Beishan orogen contains ca. 1.55–1.35 Ga basement metamorphic rocks and ca. 970–866 Ma granitoids generated during a regional tectono-magmatic event. Ca. 500–450 Ma crustal thickening and HP metamorphism may have been related to regional contraction in the South Beishan orogen. Ca. 900–800 Ma protoliths experienced eclogite-facies metamorphism (~1.2–2.1 GPa and ~700–800 °C) in thickened lower crust. These HP rocks were subsequently exhumed after ca. 450 Ma to mid-crustal depths in the footwall of a regional detachment fault during southeast-northwest–oriented crustal extension, possibly as the result of roll-back of a subducted oceanic slab. Prior to ca. 438 Ma, north-south–oriented contraction resulted in isoclinal folding of the detachment fault and HP rocks. Following this contractional phase in the middle Mesozoic, the South Beishan orogen experienced thrusting interpreted to be the response to the closure of the Tethyan and Paleo-Asian Ocean domains. This contractional phase was followed by late Mesozoic extension and subsequent surface erosion that controlled exhumation of the HP rocks.